1. Field of the Invention
The present invention relates to the mass production of concrete products such as paving stones, slabs, retaining wall units and all types of blocks, and in particular to methods and apparatus for highly controlled color distribution and blending within the face mix of concrete paving stones, but not limited to these.
2. Description of the Related Art
Natural stone has long been an attractive material for use in hardscape and masonry construction. However, owing to the high cost of natural stone, it is known to mix pigmented semi-dry concrete mixes in a mold to form a wide range of products, and in particular those often referred to as paving stones, that emulate the appearance and texture of natural stone. Such paving stones, an example of which is shown at 10 in FIG. 1, include a first “coarse mix” layer 12 made of a coarse semi-dry concrete mix having good structural properties, and a second “face mix” layer 14 which is visible as the upper surface in the finished product, and ideally has a mottled and random colored appearance approximating that of natural stone. The coarse mix layer is typically about 60 mm to 100 mm thick, and the face mix layer is typically about 5 mm to 8 mm thick.
A conventional machine 20 for mass producing paving stones is shown in FIG. 2. In general, the paving stones are formed in a plurality of molds 22 which pass through a loading zone 24 where the coarse mix and face mix are gravity fed in successive layers into the mold and packed down with a tamper and assisted by vertical vibration 26 to form the finished paving stones. Each mold has dividers to divide the coarse and face mix into the desired number and shape of paving stones within the mold.
On the coarse mix side 28, the semi-dry concrete and color pigment that form the semi-dry concrete mix are loaded into a large hopper 30. Hopper 30 supplies the coarse mix to a feedbox 32, which is mounted for horizontal travel between a first position under the hopper 30 where it receives the coarse mix and a second position over the mold 22 to be filled within the loading zone.
The structures on the face mix side 34 in conventional color blending machines generally mirror the structures on the coarse mix side. One or more hoppers 36 containing semi-dry concrete mix of differing colors supply a feedbox 38, which is mounted for horizontal travel between a first position under the face mix hopper and a second position over the mold to be filled in the loading zone. The face mix feedbox 38 travels into position and loads the face mix after the coarse mix feedbox 32 has loaded the coarse mix. The tamper then compacts the semi-dry concrete mix in the mold with the assistance of vertical vibration from the table under the mold and then the compacted product is ejected from the mold on to the production board and transported to the curing area where it hardens within a typical time of 24 hours.
It is desirable to replicate the dappled and random coloring of natural stone as closely as possible in each paving stone within a mold, and across a plurality of molds. This difficulty has not been adequately addressed in a cost effective prior art solution.
It is known to premix various colored semi-dry concrete mixes in the hopper prior to introduction of the mix into the feedbox. For example, the hopper may include stationary or movable gates for directing the inlet flow of each colored semi-dry concrete mix to one side or another of the hopper. U.S. Pat. No. 6,461,552 discloses a hopper having horizontal baffles. Concrete mixes of different colors are initially layered on top of a baffle. As the baffle is laterally withdrawn, the respective layers blend as they fall to the bottom of the hopper.
Such prior art solutions provide very little control over the degree of blending of the different colored semi-dry concrete mixes, and do not supply face mix to the feedbox in a manner that the feedbox then evenly distributes the different colors to give the desired dappled and random colored appearance.
Blending also takes place within the feedbox after transfer from the hopper. However, a further typical problem on the face mix side is that the semi-dry mix remains in the feedbox for to many production cycles and gets agitated to the point of becoming a homogeneous color. Face mix feedboxes are generally the same size as the coarse mix feedboxes. Each coarse mix feedbox typically holds enough coarse mix to fill two molds before it needs to be refilled. However, as each paving stone is made up of predominantly coarse mix, the face mix feedbox empties much more slowly, and it is common for a given supply of face mix to remain in the feedbox for twenty or so cycles before it needs to be refilled. Remaining in the feedbox for this many cycles, whatever distinct colors were initially loaded into the feedbox tend to mix with each other and become a homogeneous color as the face mix feedbox jostles back and forth over successive molds. Thus, the desired dappled, many colored appearance of the paving stones is lost.
U.S. Pat. No. 6,382,947 attempts to control the makeup of the concrete mix in the feedbox by providing three separate hoppers over the feedbox, each having a distinct colored semi-dry concrete mix. The feedbox is loaded as it passes beneath the respective hoppers. This solution tends to layer the colored semi-dry concrete mix in the feedbox, and still does not provide any significant control over the composition and distribution of the concrete color blend in the feedbox. Moreover, loading the feedbox from three separate hoppers is time consuming.
A further shortcoming of the prior art is shown in FIG. 2A. In conventional color blending machines having relatively large face mix feedboxes, the entire face mix feedbox passes over the portion of the mold nearest the face mix feedbox, and this nearest portion receives colored semi-dry concrete mix from the entire feedbox. However, the sweep of the feedbox over the mold continues only until the far edge of the feedbox reaches the far end of the mold, at which point the feedbox returns. As conventional feedboxes are relatively large, the portion of the mold farthest from the face mix feedbox only receives semi-dry concrete mix from a portion of the feedbox. Semi-dry concrete mix from a portion of the feedbox above the arrows in FIG. 2A never reaches the farthest portion of the mold. Thus, paving stones from the farthest portion of the mold tend to have a different appearance than paving stones from other portions of the mold.
It would thus be advantageous to precisely control the face mix color composition and distribution loaded into a mold to evenly distribute the semi-dry concrete mix, and to provide colors in each paving stone in a controlled percentage and in the dappled and random coloring of natural stone.